1. Field of the Invention
The present invention relates to an echo canceller, and more particularly to an echo canceller applicable to, for example, a telephone device using one or more microphones having the directivity thereof variable to capture the voice.
2. Description of the Background Art
Recently, as with the audio conferencing system, the mode of communications becomes popular which exploits a sound capturing system, such as a microphone array, having its directivity of sound capturing settable to a specific direction.
Among the solutions that control the sound capturing directivity by the microphone array, there is a sound capturing device disclosed by Japanese patent laid-open publication No. 2008-141593 to Hioka et al. Hioka et al., teaches the sound capturing device adapted for controlling the sound capturing directivity by delaying and summing voice signals obtained from plural microphones so that these voice signals will be in phase with one another.
Further, among the communications systems employing microphones having the sound capturing directivity, there is a system that has the function of detecting the direction of the sound source and automatically sets the sound capturing directivity into alignment with the direction towards the sound source in order to trace the movement of the sound source. An example of such a conventional solution is the method described in Japanese patent laid-open publication No. 304906/1999 to Tanaka. In the method taught by Tanaka, the function of tracking the sound source in its movement consists in calculating the cross correlation of signals acquired by plural microphones with the delay amount varied, and determining the delay amount when the cross correlation is maximum so as to estimatively locate the sound source from the information on the positions of the microphones and the delay amount. In order to set the directivity into alignment with the estimated direction of the sound wave, the delay amount to be used in the delaying and summation is set to the value of the delay amount thus found. By this function, when a new speaker, or talker, utters during the conference, the sound capturing directivity may automatically be set towards the new speaker, thus allowing the uttered contents to be transmitted clearly.
In order to accomplish bidirectional voice communications while howling is suppressed, the use of an echo canceller is necessary. As taught by Japanese patent laid-open publication No. 2008-219088 to Takahashi, paragraphs 0019-0021, the echo canceller suffers a problem that, if an echo path varies, the sound quality is deteriorated.
Viz., the echo canceller suffers from the difficulty that the sound quality may be deteriorated if the echo path has changed. That may also be caused when the directivity of the microphone array is controlled to render the echo path varying. With the conventional solution of detecting the echo path changing, it is necessary to carry out a sort of statistic processing, such as observing the variations in the convergence characteristics of the echo canceller or observing the specific values of the cross correlation matrix, with the result that time required in detecting the echo path variations is protracted.
To overcome the fluctuations in the echo path, the step gain of the adaptive filter is to be simply increased in response to the path fluctuations detected to accelerate the adaptive operation of the adaptive filter, thus affecting the operation of the adaptive filter. In this solution, the convergence is accelerated, indeed. However, this solution suffers a problem that the adaptive filter may sometimes not converge to its optimum value.